The present invention relates to a drive control apparatus for a brushless motor that performs duty ratio control of the brushless motor by modulating the pulse width of a drive pulse in correspondence to the setting for the rotation rate.
Methods for controlling the speed of a brushless motor include the one disclosed in Japanese Unexamined Patent Publication No. H 5-34781, which implements speed control by changing the pulse width of a drive pulse supplied to the exciting coils in order to apply a rotating magnetic field to the rotor through pulse-width modulation (PWM), for instance. In this pulse-width modulation, a triangular wave having a specific frequency is compared with a threshold value corresponding to the rotation rate that has been set, and if the rotation rate setting is smaller, the pulse width (duty ratio) of the drive pulse is reduced to lower the rotating speed of the rotor, whereas if the rotation rate setting is larger, the pulse width (duty ratio) of the drive pulse is increased to raise the rotating speed of the rotor.
However, it has been confirmed that a rush current Ia, as indicated with the one-point chain line in FIG. 7(c), is generated at the time of a rise of the source if the source is cut off for a short period of time for any reason in a motor that is rotating. This phenomenon occurs because, when the source is cut off for a short time, the voltage (the voltage at the threshold value and the like) in the signal systems that form the drive pulse does not fall off immediately due to the influence of the capacitative elements in the circuit, and when the source rises again, the motor tries to start rotating abruptly at a duty ratio level as high as that before the cut-off. If the source is cut off for a long time, on the other hand, the individual signal systems enter states similar to their initial states and when the source rises again it will be similar to a regular startup. The generation of such a rush current greatly affects the setting of the maximum rated current for the switching elements (FETs) that switch on / off the power supply to the exciting coils.
In addition, at a startup of the motor, i.e., when the motor that has been in a stopped state is to be started up, if the triangular wave and the threshold value corresponding to the rotation rate setting are simply compared, as in the prior art, to form a drive pulse, the motor will try to start rotating abruptly at a pulse width that corresponds to the rotation rate setting at the time of power up. Consequently, power is supplied to the exciting coils wound around the stator suddenly, which causes immediate repulsion from, and attraction to the permanent magnets of the rotor, to distort various portions of the motor, resulting in magnetic noise being generated.
Furthermore, it has been confirmed that when the value of the setting for the rotation rate is suddenly increased, a strong magnetic field is generated at the exciting coils, resulting in an increase in motor noise.
The common problem in the phenomena described above is that when there is a demand for a sudden upward change in the rotation rate of the rotor, the performance of the motor deteriorates due to the generation of a rush current, with abnormal noise and the like.